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FIGURE 2 Effects of pH and Pco2 (i.e., Bohr effect) and temperature on the position of the O2-hemoglobin equilibrium curve. The "physiologic" curve connects the arterial (a) and mixed-venous points (V) so the in vivo curve is steeper than the standard curve at pH = 7.4. (After Roughton, Chap. 31 in Fenn and Rahn, eds., Handbook of physiology, Section 3, Respiration. Bethesda, MD: American Physiological Society, 1964.)

affinity to increase (decreased P50), and increased PCO2 causes Hb-O2 affinity to decrease (increased P50). As described later, pH decreases when PCO2 increases and vice versa, and pH changes explain most of the Bohr effect with Pco2 changes in blood. H+ binds to histidine residues in hemoglobin molecules, and this changes the conformation of hemoglobin and the ability of heme sites to bind O2. However, CO2 also has a small independent effect on Hb-O2 affinity if pH is held constant. The physiologic advantage of the Bohr effect is that it facilitates O2 loading in the lungs, where CO2 is low and pH is high (see later discussion). In muscles, the opposite occurs and increased CO2 causes pH to decrease and facilitates O2 unloading from hemoglobin to the tissues.

The affect of temperature on Hb-O2 affinity also has physiologic advantages. Warm temperatures in intensely exercising muscles will increase P50, and decrease Hb-O2 affinity to facilitate O2 unloading to tissues.

A physiologic O2-blood equilibrium curve can be defined as the curve showing the change in blood O2 concentration when Po2 decreases from arterial to venous levels in the tissues or increases in the opposite direction in the lungs. Figure 2 shows how the increase in PCO2, decrease in pH, and potential increase in temperature between arterial and venous points makes the physiologic curve steeper than individual curves. This is an advantage for gas exchange because it increases the change in O2 concentration for a given change in PO2, thereby increasing O2 uptake or delivery. (The consequences of the shape of the O2-blood equilibrium curve for gas exchange are explained in more detail in Chapter 21.)

Hb-O2 affinity is also affected by organic phosphates, with 2,3-diphosphoglycerate (2,3-DPG) being most important in humans. 2,3-DPG is produced during glycolysis in red blood cells, and increases P50 by interacting with hemoglobin beta chains to decrease their O2-binding affinity. Physiologic stimuli that lead to enhanced O2 delivery, for example, chronic decreases in blood PO2 levels, typically increase the concentration of 2,3-DPG and promote O2 delivery to tissues. In blood stored in blood banks, 2,3-DPG is generally decreased and the increased Hb-O2 affinity can lead to problems in O2 delivery after blood transfusion.

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